An international group of four urban explorers hire a local guide, Kris (Max Riemelt), to lead them into the maze of hidden tunnels under Berlin. When Kris has a bad fall, two of the girls frantically set off to seek help while Denis (Nick Eversman), the young American, stays behind. Armin (Klaus Stiglmeier), a former East German border guard suddenly appears from nowhere. Out of sheer desperation, Denis allows Armin to lead them to safety and it is at this moment that Denis realizes he has just made the biggest mistake of his life!